This invention relates in general to devices for directing energy and, more particularly, to a hybrid coupler which is part of an integrated circuit and which is capable of directing microwaves or other energy.
Passive power couplers are a fundamental type of integrated circuit device used in many high frequency signal processing systems, such as microwave systems. Applications include balanced mixers, balanced amplifiers, phase shifters, attenuators, modulators, discriminators, and measurement bridges. An ideal hybrid coupler is a junction having four ports, which are commonly known as the incident port, the direct port, the coupled port, and the isolated port. Energy of a wave applied to the incident port is supplied to each of the direct port and the coupled port, but not to the isolated port. The amount of the input energy coupled to each of the direct and coupled ports may be about the same, or may differ according to some selected proportion.
In recent years, there has been a significant increase in the typical operating frequencies of leading edge systems which use these hybrid couplers. In particular, typical operating frequencies have increased by factors in the range of 3 to 6, for example from a typical frequency of about 3 GHz to a typical frequency in the range of about 10-18 GHz. As these frequencies have increased, it has been necessary for the size of the couplers to decrease. As a result, a broadband microwave hybrid coupler typically requires small dimensions within the coupled structure, in order to achieve tight coupling across a relatively wide frequency range. These small dimensions are commonly achieved using known types of opto-lithographic techniques, which are commonly referred to in the art as thin film technology. While couplers made with thin film technology have been generally adequate for the their intended purposes, they have not been satisfactory in all respects.
One aspect of this is that the fabrication of couplers using thin film technology involves an undesirably high cost. One less expensive fabrication technique used for other types of integrated circuits involves screen printing techniques rather than opto-lithographic techniques, and is commonly known in the art as thick film processing. However, while thick film technology is generally cheaper, the fabrication tolerances are looser for thick film technology than for thin film technology. Consequently, because small and accurate dimensions have been needed in pre-existing high-frequency coupler designs, the industry has generally considered it impractical to implement hybrid couplers using thick film techniques, especially for high-frequency applications such as microwave systems.
From the foregoing, it may be appreciated that a need has arisen for a hybrid coupler which can be made by thick film techniques, with good production yields and with good performance characteristics. According to one form of the present invention, an apparatus is provided to address this need, and involves a coupler which includes: a thick film dielectric layer having first and second sides; a thick film first strip made of an electrically conductive material and disposed on the first side of the dielectric layer; a thick film second strip made of an electrically conductive material and disposed on the first side of the dielectric layer, the first and second strips extending approximately parallel to each other; and a thick film shield made of an electrically conductive material and disposed on the second side of the dielectric layer in alignment with the first and second strips.
According to a different form of the present invention, a method of making a coupler involves: forming a dielectric layer using a thick film technique, the dielectric layer having first and second sides; forming a first strip which is electrically conductive using a thick film technique, the first strip being disposed on the first side of the dielectric layer; forming a second strip which is electrically conductive using a thick film technique, the second strip being disposed on the first side of the dielectric layer, and the first and second strips extending approximately parallel to each other; and forming a shield which is electrically conductive using a thick film technique, the shield being disposed on the second side of the dielectric layer in alignment with the first and second strips.